1. Field of the Invention
The present invention relates to a plastic-encapsulated semiconductor device.
2. Description of the Related Art
As the integration level of semiconductor devices increases, the packages forming the semiconductor devices become undesirably larger. Because these packages are encapsulated, a thinner encapsulation is desirable in order to reduce mounting space.
The recent semiconductor devices, such as ASIC (Application specific integrated circuits), generate a large amount of heat during operation because of their high integration level and high operation speed. Therefore, the latest semiconductor devices require a new resin encapsulant having a high thermal conductivity for easy heat dissipation.
Resin-encapsulated semiconductor devices greatly vary in package type and the size of the production lots for each package type is limited. Therefore, a suitable resin encapsulation for various types of semiconductor devices is required that is economical for small production lots.
Conventional resin-encapsulated semiconductor devices are encapsulated by a transfer molding process. Since the transfer molding process requires batch processing, it is difficult to carry out encapsulation by in-line processing. Therefore, the transfer molding process is not suitable for manufacturing various types of semiconductor devices in small lots.
Japanese patent disclosure KOKAI 2-257662 and U.S. Pat. application Ser. No. 40,532 filed on Mar. 30, 1993 disclose an improved method, which meets the above requirements, using a resin sheet. In the method, a resin sheet is disposed on a semiconductor chip and then it is pressed and heated by a mold. This method is applicable for in-line processing. Further, a thinner encapsulation can be achieved using the disclosed method.
However, some problems still remain. In general, it is desirable to add certain additives to the resin sheet so as to improve a particular property of the encapsulant. However, there are some properties that are advantageous in one aspect, but disadvantageous in another. Moreover, some additives have additional properties that are disadvantageous. As a result, it is difficult to obtain the advantageous properties of an additive without also obtaining the disadvantageous properties.
For example, a mold releasing agent may be added to the resin sheet in order to prevent a residual flash of the cured resin from remaining on an encapsulant. However, the addition of the mold releasing agent reduces the adhesion strength of the encapsulant to the semiconductor chip on which the encapsulation is formed. The reduction of the adhesion strength causes a poor moisture resistance because of the increased probability of cracking between the chip and the resin.
A method has been proposed in Japanese patent disclosure KOKAI 2-257662, which uses a laminated sheet. The laminated sheet includes an uncured resin sheet and a sheet, such as metal, ceramic or cured resin, that does not melt in molding processing. Therefore, this eliminates the problem of releasing the resin from the mold. However, since different sheets tend to delaminate, reliability is sacrificed. In U.S. Pat. No. 4,680,617, a lamination of a prepreg and a glass-fiber layer is described. However, this laminated sheet does not release well from the mold.
An inorganic filler having high thermal conductivity is usually added to a resin to improve heat dissipation. However, the addition of the filler increases the viscosity of the resin. Therefore, a resin containing a large amount of the fillers for high thermal conductivity is too viscous for molding. Further, the addition of a large amount of the fillers damages the semiconductor chip and bonding wires.
An idea has been proposed in Japanese patent disclosure KOKAI 5-175264, which uses a sheet including a low-stress low-viscous resin on an active surface of a semiconductor chip, and a high-thermal-conductivity high-viscous strong resin on the opposite surface of the chip. However, Japanese patent disclosure KOKAI 5-175264 does not describe how to realize such a sheet. However, since different resins tend to delaminate, reliability is low.
Another additive having both advantageous and disadvantageous properties is a curing accelerator. The curing accelerator can improve productivity. However, the addition of a large amount of the accelerator causes residual amounts of the accelerator to remain in the resin. The residual accelerators corrode metal lines of the semiconductor chip. Also, flame retardant additives have the same tendency.
As above-mentioned, there are many properties of a resin or requirements for encapsulation. However, since each of additives for some property may be harmful against other properties, both of conflicting properties can not be improved.